The bombing took place inside a police reserve unit dining room in Trinkot, in Uruzgan province, on Friday afternoon, when dozens of officers were having lunch ahead of the important Friday prayer session.

"Police were having lunch when a man with police uniform detonated his suicide vest, killing 12 police and wounding five. Four of those are in critical condition," said Farid Haeel, a spokesman for the provincial police chief.

The Taliban have promised to attack Afghan forces as well as NATO-led forces still in the country, and earlier this year said insider attacks would be a central tactic used over the summer fighting months.

Many insider attacks are so-called green-on-blue, in which uniformed Afghan police or soldiers attack Western troops, around 100,000 of which remain in the country ahead of a combat drawdown winding up next year.

But most of the attacks are green-on-green, in which members of the 350,000-strong Afghan National Security Forces attack their own comrades. Figures for such attacks are, however, difficult to establish.

Two Afghan police shot dead seven officers in May as they slept in their beds.

Haeel said an investigation was underway to find out how the bomber on Friday penetrated past security measures in place to guard against insider attacks.

The last month has been a particularly bloody period for Afghan police, with 299 officers killed and 617 wounded between mid-May and mid-June, Interior Ministry spokesman Sediq Sediqqi said on Monday.

Violence is escalating across Afghanistan as most NATO-led combat troops prepare to leave by the end of 2014, leaving behind a reduced force of Western advisers and special forces, the precise size of which is still being negotiated.

Nuristan, neigbouring Pakistan, is a insurgent hotbed and resupply route which has been a long-running problem for both Afghan and NATO-led forces, which have struggled to even gain a foothold in the mountainous and heavily-forested area. (Reporting by Ismail Sameem, Writing by Mirwais Harooni, Editing by Rob Taylor and Ron Popeski)